This invention relates orthogonal frequency division multiplexing (OFDM) based spread spectrum multiple access such as may be used in wireless, and other, communication systems.
It is desired that wireless communication systems be as efficient as possible to maximize a) the number of users that can be served and b) the data rates, if data service is provided. Wireless systems are shared media systems, i.e., there is a fixed available bandwidth that must be shared among all the users of the system. These systems are often implemented as so-called xe2x80x9ccellularxe2x80x9d systems, where the covered territory is divided into separate cells, and each cell is served by a base station.
It is well known in the art that the two particularly desirable features of a cellular wireless system are 1) that the intracell interference, i.e., interference experienced by one user that is caused by other users that are within the same cell as that user, be as small as possible, and 2) that the intercell interference, i.e., interference experienced by one user that is caused by other users that are in cells other than the one in which the user is located, is averaged across all users in neighboring cells. Most prior art digital cellular systems are time division multiple access (TDMA) systems, such as group special mobile (GSM)-, intermediate standard (IS)-136-, or IS-54-based systems, or they are code division multiple access (CDMA) systems, e.g., IS-95 based systems.
In prior art narrow band TDMA systems neighboring base stations use different, e.g., non-overlapping, parts of the available spectrum. However, bases stations that are sufficiently far away from each other to avoid substantial interference between them, i.e., non-neighboring base stations, may use the same parts of the available spectrum. Notwithstanding such spectrum reuse, the spectrum available for use in each cell is a small part of the total available spectrum. Each user in a cell has its own unique frequency band and time slot combination, and hence TDMA systems have no intracell interference, i.e., they have the first desirable feature of cellular wireless systems. However, TDMA systems do not have the second desirable feature, in that a given user only interferes with a small number of users outside the cell, so that spectral reuse is based on worst case interference rather than average interference. As a result, the system has a low xe2x80x9cspectralxe2x80x9d efficiency.
In prior art direct sequence (DS)-CDMA systems the entire bandwidth is used by each base station but each base station uses a different spreading code. Such CDMA systems promise higher spectral efficiency than narrow band TDMA systems. Thus, CDMA systems have the second desirable feature of a cellular wireless system. However, CDMA systems do not have the first desirable feature of a cellular wireless system because although the signals transmitted from the base station within a cell are orthogonal, because of channel dispersion, the signals received at a receiver are not necessarily orthogonal. This results in interference between users within the same cell.
Proposed prior art frequency hopping (FH)-CDMA systems are very similar to narrow band TDMA systems, except that they employ frequency hopping to also obtain the second desirable feature of a cellular wireless system. In particular each transmitter transmits a narrow band signal, and periodically changes the carrier frequency to achieve the frequency hopping. However, disadvantageously, such hopping is relatively slow, reducing the amount of averaging that can be achieved for a given delay in the transmission path that the system can tolerate.
U.S. Pat. No. 5,410,538 issued to Roche et al. on Apr. 25, 1995 discloses a multi-tone CDMA system. Such a system is essentially an OFDM system that eliminates intracell interference by insuring that the received signals within a cell are orthogonal. Thus, the Roche et al. system has both desirable features of a cellular wireless system. However, the Roche et al. system partitions the spectrum into a large number of tones, which makes the system very susceptible to Doppler shifts in mobile systems. Also, because each mobile user transmits on a large number of tones, the peak-to-average ratio of the mobile transmitter is very high, resulting in poor power efficiency at the mobile station, which is disadvantageous in that power is often a limited resource in the mobile station.
U.S. Pat. No. 5,548,582 issued to Brajal et al. on Aug. 20, 1996 discloses a general wide-band orthogonal frequency division multiplexing (OFDM) based spread spectrum multiple access.
We have recognized in United States Patent Application Serial No. (Case Laroia 9-1-1) that the Brajal et al. system is not optimized for use in a cellular system in that there is no teaching therein how to optimize a) the hopping pattern, b) the tone assignment, or c) the bandwidth reuse. We have further recognized that optimizing these factors, individually and/or collectively, is important to obtain a spectrally efficient system, i.e., a system that has the two particularly desirable features of a cellular wireless system. In particular, we disclosed in United States Patent Application Serial No. (Case Laroia 9-1-1) dividing the entire bandwidth into orthogonal tones, and reusing all of the orthogonal tones in each cell. To reduce peak-to-average ratio at the mobile transmitter, low bit rate user, such as a voice user, is allocated preferably a single one, but no more than a very small number, of the orthogonal tones for use in communicating with the base station. Data users are similarly allocated tones for data communication. However, the number of tones assigned for each data particular user is a function of the data rate for that user. The tone assignment for a given user is not always the same within the available band, but instead the tones assigned to each user are hopped over time.
A tone hopping pattern was disclosed that achieves maximum frequency diversity and averages the intercell interference, e.g., using a pattern that is a function of a mutually orthogonal latin square. More specifically, in the downlink, i.e., in the channel from the base station to the mobile station, the tones assigned to each user are change relatively rapidly, e.g., from symbol to symbol, i.e., the user fast xe2x80x9chopsxe2x80x9d from one tone to another. However, in the uplink, i.e., in the channel from the mobile station to the base station, although fast hopping is possible, preferably slow hopping is employed to allow efficient modulation of the uplink signal. However, when slow hopping is used in the uplink, it is necessary to employ additional techniques, such as interleaving, to compensate for the reduction in the intercell interference averaging effect.
We have recognized that notwithstanding the foregoing advancements, additional improvements are yet necessary to achieve spectrally efficient system, i.e., a system that has the two particularly desirable features of a cellular wireless system. One such improvement, in accordance with the principles of the invention, is the use of offsetting between cells, and in particular, the use of tone offsetting and time offsetting. More specifically, in accordance with an aspect of the invention, frequencies that define the tone set of one cell is offset from the frequencies that define the tone set of at least one adjacent cell. In other words, if a first base station is using tones F1, F2, . . . , FN within a frequency band, then a second base station adjacent to the first base station uses tones F1+xcex94f, F2+xcex94f . . . , FN+xcex94f within the same frequency band. In accordance with another aspect of the invention, the symbol timing of the base-station of one cell is offset from the symbol timing of the base-station of an adjacent cell. Thus, if a first base station starts successive symbols at times T1, T2, and T3, then a second base station adjacent to the first base station starts its respective corresponding successive symbols at times T1+xcex94t, T2+xcex94t, and T3+xcex94t. For example, in a cellular system with hexagonally shaped cells, the tone sets of two adjacent cells are offset by ⅓ of the spacing between adjacent tones, and the symbol timings of two adjacent cells are offset by ⅓ of a symbol period. Advantageously, the intercell interference is more uniformly distributed among users in a cell.